Due to the recent serious environmental problem, land boilers for power generation are required to reduce NOx in flue gas, and measures are often taken from the viewpoint of combustion technology, such as two-stage combustion with relatively low cost.
However, two-stage combustion makes the combustion atmosphere in a boiler furnace more reducing, causing a problem of corrosion of boiler components such as water wall tubes. The problem with the boiler furnace water wall tubes today is sulfidation corrosion due to a reducing atmosphere. Since this corrosion causes damage over a wide range in the furnace, it requires a lot of time and labor to identify the corrosion.
As shown in a boiler schematic diagram of FIG. 1, a boiler furnace 1 is generally constructed of four water walls 3 and the inside is burned by a burner. Inside the water walls 3, multiple water wall tubes 4 extend in the up-down direction (vertical direction) and are arranged adjacent to each other by welding or in contact with each other. Alternatively, the water walls 3 themselves may be made up of the multiple water wall tubes 4 extending in the up-down direction (vertical direction) and welded adjacent to each other. Then, water is poured into the water wall tubes 4 and heated therein to produce steam. Such a boiler furnace 1 is, for example, 70 m high, and the width of its side wall is about 30 m, with more than 100 water wall tubes 4 installed on one side wall face.
Therefore, in order to identify a location where damage from sulfidation corrosion is developing, visual inspection (to check for surface unevenness visually), palpation (to detect surface unevenness with bare hands), and further an ultrasonic thickness gauge are used. However, since the area to be inspected is large, it takes time and labor to inspect the area. Further, visual inspection is affected by the inspector's subjective view, and there is a problem that the results vary widely.
The ultrasonic thickness measurement is quantitatively reliable, but since it is a point measurement to measure wall thickness at a fixed point, it is not suitable for measuring general corrosion conditions of water wall tubes.
On the other hand, Patent Document 1 (Japanese Patent Application Laid-Open No. 09-257714) teaches an inspection device for inspecting surface abrasion conditions of water wall tubes in a boiler furnace.
In Patent Document 1, as shown in FIG. 15, a heat transfer tube group placed horizontally in the boiler is made up of a heat transfer tube bank 02 and a heat transfer tube cavity portion 03, where an inspection device 04 is placed between the heat transfer tube bank 02 and the heat transfer tube cavity portion 03. The inspection device 04 includes a machine body 05, a flexible arm 06 provided on the machine body 05 to be inserted into the heat transfer tube bank 02, an end fixture 07 attached to the tip of the flexible arm 06, and an inspection unit 09 including an optical detection sensor movable in the up-down direction along a guide mechanism 08 provided in the flexible arm 06.
In the case of inspecting the outer surfaces of heat transfer tubes 010, the flexible arm 06 is inserted into the heat transfer tube bank 02, and when it reaches the bottom of the heat transfer tube bank 02, the end fixture 07 at the tip of the flexible arm 06 is actuated to fix the tip end. Then, a heat transfer tube alignment mechanism 011 provided in the flexible arm 06 is inflated pneumatically or mechanically to push the heat transfer tubes 010 to spread out laterally in order to correct misalignment of heat transfer tube arrays. After that, the inspection unit 09 is moved up and down along the flexible arm 06 to carry out inspection during the vertical movement.
However, the inspection device shown in Patent Document 1 is an inspection device for the heat transfer tubes 010 in the boiler furnace constructed of the heat transfer tube bank 02 and the heat transfer tube cavity portion 03 when heat transfer tube groups are placed horizontally in the boiler, and it cannot be applied to a structure where water wall tubes are arranged adjacent to each other in a longitudinal direction (vertical direction) along boiler inner wall surfaces.
Further, Patent Document 1 only teaches that the inspection unit 09 can measure the wall thicknesses of the heat transfer tubes 010 in a non-contact manner, and mention is made of neither a specific technique for calculating the wall thicknesses of the heat transfer tubes nor calculation of the amount of reduced wall thickness.    [Patent Document 1] Japanese Patent Application Laid-Open No. 9-257714